In the detection, or sensing of the presence and extent of atomic particles, it has been the practice in the art to use solid state semiconductor structures that can produce a signal resulting from hole-electron pairs that result, when the atomic particles in passing through the semiconductor, engage in primary or secondary collisions. The hole-electron pairs are sensed by sweeping them into external contacts before they can recombine or be trapped.
There are several characteristics of radiation detection that result in special structural features in these devices. The propelling energy of the particles may be so great that relatively long distances may be required for absorbtion. The electron-hole pairs produced by an atomic particle collision must be distinguishable from any other carriers present due to any crystal imperfection or thermal carrier generation. Fields employed in sweeping the collision generated carriers into external contacts must be high due to the long absorbtion distances. The resulting charge is converted into a pulse, with the narrower the pulse, the better the time resolution and therefore a higher counting rate can be achieved. The total amount of charge in the pulse should be proportional to the energy of the exciting radiation. Thus, events that prevent extracting all of the charge within the pulse will degrade energy resolution.
In efforts to accommodate the needed features, the semiconductor structure generally has a large region, of intrinsic or semiinsulating conductivity, that is as depleted of carriers from any source other than an atomic particle collision, as the state of the technology will provide. In some cases several technological compromises are made.
In the case of the monoatomic semiconductors such as germanium and silicon since each has a relatively small band gap which produces more thermally generated carriers and limits the magnitude of any field applied across the absorbtion region for sweeping the collision produced carriers into the contacts, low, usually 77 degrees kelvin, liquid nitrogen temperatures are used to reduce thermally generated carriers. A requirement for such a low temperature in operation has a limiting effect on the places the device can be used. Recent efforts in the art have been in the use of the wider bandgap semiconductor material gallium arsenide in an effort to produce a radiation detector structure that can be operated in the 300 degree Kelvin or room temperature range. A serious limitation has been difficulty in producing thick enough and more perfectly crystalline absorbtion regions.
In an article by Alexiev et al. published in Nuclear Instruments & Methods in Physics Research A 317 (1992) Pages 111-115 there is reported the construction of room temperature radiation detectors of gallium arsenide wherein, using liquid phase epitaxial growth, 200 micro meter thick absorbtion regions are formed. The radiation enters the absorbtion region through a surface barrier rectifying contact.
Further efforts using the material GaAs are reported by Mc Gregor et al. published in Nuclear Instruments & Methods in Physics Research A 317 (1992) Pages 487-492, wherein detectors, using a Schottky barrier contact, and having absorbtion regions of increasing thicknesses of up to 250 micrometers, that were cut from pulled crystals, were found to exhibit decreasing resolution, as the thickness increased.
A need is developing in the radiation detector art for the ability to provide detector structures that operate at 300 degrees Kelvin and which have substantially greater thickness absorbtion regions without degrading energy resolution.